Industrial processes can suffer large financial losses following power disruptions. A typical method of restoring power service to loads is to reclose tripped circuit breaker(s), either manually or automatically, after some preset interval. Following reclosing there is a large current inrush to the load—often well in excess of the current that served the loads prior to the outage or interruption. This inrush current can overstress and degrade the reliability of the breaker/recloser hardware and can reduce their lifetime. The inrush current can also damage the loads themselves. Moreover, when power is resupplied by the reclosing of circuit breakers, all motors of the industrial loads can attempt to start at the same time. Motor starting characteristics can require up to ten times normal operating current during start up. This large current demand can result in voltage depression which can quickly cascade into total voltage collapse. Power companies can impose limits on the amount of current drawn and the maximum number of starts during a given time period, such as over the course of an hour or a day, to limit such voltage collapse.
Service restoration after an outage or power disruption can involve manual restarts to ensure safety during process restarts. The manual restart process can take a significant period of time (e.g. 45 minutes). Moreover, the restarting sequence has to be considered and timed to reduce inrush current and voltage fluctuations. Soft-starters have been used to limit inrush currents for larger motors. Installment of soft-starters can avoid penalties imposed by power companies on the amount of current drawn and the maximum number starts per time period. However, soft-starters are not always a viable solution considering the added costs of the soft-starters and the potential for injecting harmonic currents into the power network, which may aggravate power factor.
Energy storage systems have been used as an auxiliary source for power networks for applications ranging from peak load support to power quality control. An energy storage system typically includes a power source (e.g. a battery source, fuel cell, generator, capacitor bank, or other suitable power source) that is coupled to the power network via an inverter. The inverter converts the power available from the power source to a suitable alternating current power for application to the industrial power network.
The potential of using an energy storage system during reclosure has not been fully realized due to grid regulations. For instance, to avoid asynchronous reclosure, energy resources may have to be disconnected and a dead-bus condition may have to be achieved prior to reconnection with the utility grid. Achieving a dead-bus condition can require extended downtime after a power outage or disruption.
Thus, a need exists for a method for service restoration of industrial loads in an industrial power network that reduces transients and process downtime. A system and method that can take advantage of the fast dynamics of an energy storage system inverter and existing protection relays would be particularly useful.